Electronic packages which include at least one thin film, flexible circuit member as an integral part thereof are known in the industry, with examples being defined in U.S. Pat. Nos. 4,849,856 (J. Funari et al) and 4,914,551 (M. Anschel et al), both of which are assigned to the same assignee as the instant invention. In a more recent patent, also assigned to the same assignee as the present invention, there is defined a more specific example of such a flexible thin film carrier. See U.S. Pat. No. 4,937,707 (D. G. McBride et al). As mentioned therein, a main objective in the design of such circuit members and electronic packages is to provide these with as high current densities as possible within a minimum of space. Such miniaturization efforts, while providing several highly advantageous features, also present various engineering problems in both the manufacture and operation of these structures. In the aforementioned U.S. Pat. Nos. 4,849,356 and 4,914,551, for example, solutions to assure effective package heat removal during operation as well as facilitating package assembly are defined. The above three patents are incorporated herein by reference.
One particular problem facing designers of electronic packages containing thin film circuit structures is the essential requirement to provide sound electrical connections between the relatively large number of extremely small conductive elements (e.g., signal and/or ground) of the flexible circuit and the respective conductor locations at the circuitized substrate (e.g., printed circuit board to which the flexible circuit (and the chip thereon) is electrically coupled. As will be defined herein, the present invention defines a method of forming such electrical connections between a flexible circuit and such a substrate for use within structures such as electronic packages in order that relatively high circuit densities are attained. As will be further defined, such connections and the resulting structure are accomplished using solder. Even more particularly, these connections are achieved without use of a separate chemical flux or the like.
As is known, soldering represents a common technique for joining various objects together, including thin film and other circuits, as well as between flexible circuits and electronic devices (chips). Typically, the conductive portions of these structures (usually metallic, e.g., copper) have been coated with a chemical flux prior to joining. Solid solder is then placed between the flux-coated objects or a flux-filled solder paste is screen-printed onto one of the conductive portions and heated to a molten state such that the molten solder coats the portions of these objects which in turn have been coated with such flux. Thereafter, the molten solder is cooled to form a joint between the two objects.
In microelectronic applications (e.g., for electronic package structures as defined herein), solder bonds are commonly used. In one technique, an electronic device (semiconductor chip) is mounted onto a packaging substrate (thin film structure) in a "flip-chip" orientation. In this orientation, the surface of the chip having contact locations is positioned facing the flexible, thin film circuit member. The contact locations on the chip and the respective thin film conductive elements are electrically connected using solder structures, also referred to as controlled collapse chip connections (C4's).
In the aforementioned, U.S. Pat. No. 5,057,969 the method defined therein represents a significant improvement in providing flexible circuit-semiconductor device connections. Use of flux has been substantially eliminated. The method defined in U.S. Pat. No. 4,937,006, also assigned to the same assignee as the present invention, represents yet another improvement over existing and other previous techniques, primarily through effective gas direction control onto the respective solder mounds which form part of the connections being formed. U.S. Pat. No. 4,937,006 is also incorporated herein by reference.
As defined below, the present invention provides another significant improvement over currently known soldering techniques for performing electrical connections in miniaturized electronic packages. The method as defined herein assures sound, effective electrical connections between extremely small circuit members, particularly those found on the package's circuit and the circuitized substrate (printed circuit board) to which it is to be bonded and electrically coupled. The defined method is relatively safe and can be adapted to mass production techniques, thus reducing the overall cost of such a final package structure.
It is believed that a method of providing electrical interconnections between two circuitized substrates (e.g., a thin film flexible circuit and a printed circuit board) possessing the several advantageous features described herein would constitute a significant advancement in the art, including particularly the art of electronic packages in which such substrates are particularly suited for use.